Integral Inductor-Susceptor

ABSTRACT

An induction heating inductor and perforated susceptor are formed as an integral unit to provide a low cost, physically stable, efficient, and easily cleaned unit.

FIELD OF THE INVENTION

An inductor coil is bonded to the surface of an electrically insulatedperforated steel susceptor to form an integral unit for inductivelycoupling energy from the inductor to the susceptor.

BACKGROUND OF THE INVENTION

Solid to liquid transformations by technology described in Lasko patentsU.S. Pat. No. 5,584,419 and U.S. Pat. No. 7,755,009 require inductorcoil forms that often impede material flow. Solid or particulate formelectrically nonconductive materials are presented to one surface of aninductively heated perforated susceptor for melt transformation uponpassing to the other side by gravity flow or mechanical pressure. Whenthe susceptor form is a disc, it acts as a face of a cylindricalcontainer for the process material. A cone form susceptor acts as aconical end of a cylindrical container. A cylinder form susceptor is aportion of the cylindrical container. These shapes are necessarily fullyradial to accomplish an evenly distributed coupling of the magneticfield. The objective of the inductor coil design for this meltingprocess is to distribute the magnetic field intensity in proportion tothe volume flow over the surface of the susceptor. Efficient transfer ofenergy to the susceptor requires placement of the individual inductorelements in close proximity to the susceptor surface. The number ofelements [off-set concentric turns or spiral turns] per unit area of thesusceptor surface is varied to distribute the magnetic field intensityand resulting energy transfer from the inductor coil to the susceptor.These variations control the influence of the inductor coil magneticfield edge effect and inter-turn deviation [flux leakage].

Sheets of industry standard staggered round hole perforated steel areused to construct susceptors of disc, cone and cylinder form. The sizeand number of perforations in the susceptor are chosen to maximize thesurface area of the susceptor for thermal conduction to the processmaterial, while restricting open area to preserve thin sheet strengthand adequate cross sectional area for even induced current flow. Thethermal conductivity and temperature variable viscosity of the processmaterial further defines the hole size. An open area of approximately50% meets this requirement for most materials. The material must flowthrough the susceptor in unimpeded volume related to the energytransferred at any point on the susceptor to impart a homogeneousmaterial temperature.

Processing different materials in the same apparatus requires purgingthe previous material with the new material. Additional surfaces ofinductor coil supports and the coil occupied area add to the volume ofmaterial lost to this process. Lesser viscosity materials in gravityflow will not adequately displace materials of greater viscosity.Removing the inductor and susceptor for chemical cleaning is not anattractive alternative. The process start and stop interval islengthened by the total thickness of the inductor coil and susceptorassembly. Because the susceptor is the material containment vessel or apart there of, support for this item in the apparatus is complicated bythe necessary close proximity position of the inductor coil.

This invention provides a method of meeting these physical andelectrical requirements by direct placement of the inductor coil on thesusceptor surface and perforating the inductor coil with axis anddiameter coincident holes. The hydraulic pressure required to passmaterial through this thermal interface is reduced to that of thesusceptor alone. The inductor coil does not need to be separatelysupported in the material flow path. Similar materials can be processedwith minor volume displacement of the previous material in theapparatus. Extraction of the integral inductor-susceptor for chemicalcleaning is made practical by requiring only the removal of anelectrical connection and striping the surface of a single unit ofsimple form.

When the adjacent inductor coil material is axis coincidentallyperforated, its electrical conducting cross section is diminished. Theresistance of the total remaining conductor cross-section must remainlow enough to support the desired amount of high frequency currenthaving electrical energy losses that are thermally transferable to theprocess material. The thickness of the inductor coil is increased topreserve the required minimum cross section.

The inductor is made integral with the susceptor by direct placement onan electrically insulated susceptor surface. This bond provides anaccurate and mechanically stable orientation of the inductor in closestproximity of the susceptor. This is achieved in one embodiment of theinvention by plating the inductor coil on one or both surfaces of aporcelain enamel coated perforated steel disc. The perforated sheetsteel disc is etched to radius the holes edges and decarburize thesurface. The entire disc surface and holes are coated with 0.009″ ofporcelain enamel. The disc is electroless copper plated, pattern masked,etched, striped, electroplated, and refired. The coefficient of thermalexpansion of the steel disc susceptor, porcelain enamel coating, andcopper overlay are close enough to maintain an effective bond fortypical maximum process temperature excursions of 400° F.

The process residency time for most thermoplastic materials is a fewseconds. Power applied at 20 to 50 watts/sq.″ will melt mostthermoplastic materials at gravity pressure on the susceptor surface.The frequency of the power applied to the inductor coil is 40 to 100KHz. The process temperature can be precisely controlled by placing athermocouple on the susceptor to signal a controller for modulating thehigh frequency power applied to the inductor.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an integral inductor/susceptor.

FIG. 2 is an isometric view of a 90° section of an integralinductor-susceptor having axis coincident perforations.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross section of the edge of a 15″ dia. 19 ga. staggeredpattern perforated sheet steel disc susceptor 1. Susceptor 1 is coatedwith 0.009″ porcelain enamel 2. Magnetic field inductor coil 3 isconstructed of 22 rectangular turns of copper alloy screen printed andplated to 0.020″ thickness on the porcelain enamel 2 surfaces.Individual inductor coil turns 4 are identified as A through D. Turns Aand B are the first and second turns of the inductor introduced at edgeHF power entry point 5. Holes in the center position turn are plated asa printed circuit via to pass current to the opposite side of susceptor1. A mirror image of inductor coil 3 is placed on the opposite side ofthe susceptor to return the current to edge HF power entry point 5. Thepolarity signs (+/−) 6 indicate the instantaneous half cycle directionof the current flow required to make the magnetic fields 7 and 8additive as intercepted by the susceptor. The field force lines 9intercept the susceptor 1 with equal intensity. All susceptor holes 10are 0.094″ diameter prior to applying porcelain enamel 2. Arrows 11indicate the flow of melting material passing through the integralinductor-susceptor. This arrangement of the coil and susceptor resultsin minimum heat energy remaining in the inductor-susceptor as power isturned off. It is most appropriate for applications where a faststart-stop of the melt flow is desirable.

FIG. 2 is a shaded isometric view of a 90° segment of a coatedperforated disc susceptor with a spiral copper coil bonded to thesurface. The individual turns 12 of the inductor coil are of differingwidth to even the magnetic field intensity profile across the disc. Theperforated disc susceptor section 13 is coated with 0.009″ thickporcelain enamel that is to too thin to depict relative to its 0.040″thickness and the individual turns 12 thickness of 0.020″. Perforationholes 14 in individual turns 12 are axis aligned with those of susceptorsection 13. Staggered hole perforated sheet steel is preferred for thisconstruction to aid in preserving individual turn cross section at allsegments of its track.

1. An integral inductor-susceptor for heating electrically nonconductivematerials that includes the following elements: a perforated susceptorhaving an electrically insulating coating; an inductor coil integrallybonded to the electrically insulated coating surface; a means ofpresenting the electrically nonconductive materials to a surface of theintegral inductor-susceptor; and a high frequency power supply to powerthe inductor coil.
 2. The integral inductor-susceptor according to claim1 that presents an entire face of a disc, cone, or cylinder to meltsolid or particulate form material.
 3. The integral inductor-susceptoraccording to claim 1 having a susceptor formed of perforated steelsheet.
 4. The integral inductor-susceptor according to claim 1 thatutilizes porcelain enamel as the susceptor insulating coating.
 5. Theintegral inductor-susceptor according to claim 1 having susceptorperforation axis coincident holes placed in said inductor coil.
 6. Theintegral inductor-susceptor according to claim 1 having said inductorcoil formed as a spiral from a peripheral electrical connection point,through holes in a center position turn, and returning to the peripheryin an opposite wound spiral on the opposite side of said susceptor. 7.The integral inductor-susceptor according to claim 1 having elementsdesigned specifically for melting thermoplastic materials.
 8. A methodof heating electrically nonconductive materials comprising the steps of:positioning said electrically nonconductive materials to contact asurface of a perforated integral inductor-susceptor; energizing saidperforated integral inductor-susceptor with a high frequency powersupply; and inductively coupling energy from the inductor to thesusceptor to heat the susceptor.
 9. The method of claim 8, furthercomprising the step of controlling a heating temperature of the integralinductor-susceptor to a temperature higher than the resulting processtemperature of said electrically non conductive materials flow.
 10. Themethod of claim 8, wherein said perforated integral inductor-susceptoris sized for melting thermoplastic material.
 11. The method of claim 8,wherein the electrically non conductive materials are placed for gravityflow.
 12. The method of claim 8, wherein the electrically non conductivematerials contact a surface of a disc, cylinder, or cone shaped saidperforated integral inductor-susceptor.
 13. The method of claim 8,wherein solid cylindrical forms or particulate forms of saidelectrically non conductive materials are positioned for heating.